At present, in the field of mobile communication systems, communication systems in operation employ CDMA (Code Division Multiple Access) as a multiple access scheme. On the other hand, a study on next generation mobile communication systems has been very active, aiming for much faster wireless communications. The 3GPP (3rd Generation Partnership Project) which develops standards for third generation mobile communication systems, for example, is working on standardization of new specifications for mobile communication systems, called LTE (Long Term Evolution) (for example, refer to 3rd Generation Partnership Project, “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access (E-UTRAN); Overall description; Stage 2 (Release 8)”, 3GPP TS36.300, 2007-06, V8.1.0.).
The next generation mobile communication systems are supposed to employ OFDMA (Orthogonal Frequency Division Multiple Access) or SC-FDMA (Single Carrier-Frequency Division Multiple Access) as a multiple access scheme. Such mobile communication systems schedule uplink data transmission from a mobile station to a base station as follows.
When the mobile station has control information and other data to transmit, the base station performs dynamic allocation of radio resources in both the frequency domain and the time domain for an uplink data channel. Then, the base station provides the mobile station with the result of the radio resource allocation. According to the result, the mobile station transmits both the control information and the other data at the allocated frequency and in the allocated time slots.
When the mobile station has only control information to transmit, on the other hand, the mobile station is not allocated any resource for the uplink data channel, and transmits the control information to the base station on an uplink control channel which is a radio resource previously set for transmission of control information. The control information which is transmitted on the uplink includes ACK (ACKnowledgement)/NACK (Negative ACKnowledgement) which is a response to data from the base station, and CQI (Channel Quality Indicator) which is a measure of the quality of downlink communication (for example, refer to 3rd Generation Partnership Project, “Physical Channels and Modulation (Release 8)”, 3GPP TS36.211, 2007-05, V1.1.0.).
By the way, the base station preferentially allocates a frequency band with the best uplink communication quality for the uplink data channel, from the available frequency band between the base station and the mobile station. Therefore, before being allocated a resource for the uplink data channel, the mobile station needs to transmit to the base station a wideband pilot signal (SRS: Sounding Reference Signal) that is used for measuring the quality of uplink communication. In this case, there arises a problem of how to multiplex control information and SRS when a same or different mobile stations transmit them simultaneously. To tackle this problem, the following multiplexing scheme has been proposed (for example, refer to 3rd Generation Partnership Project, “Multiplexing of Sounding RS and PUCCH”, 3GPP TSG-RAN WG1 #49bis R1-072756, 2007-6).
FIG. 21 illustrates an example of uplink signals including SRS. In this example of FIG. 21, ACK is transmitted as control information with two frequency bands as uplink control channels i and j. The mobile station is permitted to use one of these uplink control channels i and j to transmit the control information. On each uplink control channel, a signal indicating control information and a pilot signal (RS (Reference Signal)) are scheduled in a predetermined order. However, in a predetermined portion of a unit time period, all frequency bands are reserved as a radio resource for SRS transmission. When transmitting SRS, the mobile station uses the reserved resource in the predetermined portion of the unit time period.
However, the time multiplexing scheme employed in the above “Multiplexing of Sounding RS and PUICCH, 3GPP TSG-RAN WG1 #49bis R1-072756, 2007-6” (3rd Generation Partnership Project) does not permit control information to be transmitted at the same time as a wideband signal to be used for measuring communication quality. Therefore, as compared with the case of not multiplexing a wideband signal and a control information signal, this scheme provides less radio resources available in every unit time period on every uplink channel. This causes problems that the quality of receiving a signal indicating control information deteriorates at a receiving apparatus (corresponding to the above-described base station on the uplink) and that the number of transmitting apparatuses (corresponding to the above-described mobile station on the uplink) which can be covered by each control channel decreases.